Light-emitting device, and lighting system, and image display using same

ABSTRACT

To enhance luminance and color rendering of a light emitting device comprising phosphors as wavelength converting material and 
     at least one semiconductor light emitting device that emits visible light, as said phosphors, are used phosphors which are one or more kinds of phosphors selected from a group consisting of oxides, oxynitrides and nitrides, and are a mixture consisting of two or more kinds of phosphors whose luminous efficiency is 35% or higher when excited by the visible light from said semiconductor light emitting device at room temperature. In addition, said mixture contains a first phosphor, and a second phosphor that is different from said first phosphor and capable of absorbing emitted light from said first phosphor, and said first phosphor is contained 85 weight % or more of said mixture of phosphors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/631,103, filed May 8, 2007 as the U.S. National Stage ofInternational Application No. PCT/JP05/011941, filed Jun. 29, 2005, thedisclosures of which are incorporated herein by reference in theirentireties. This application claims priority to Japanese PatentApplication Number 2004-194509, filed Jun. 30, 2004, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to a light emitting device, and lighting system,display using the same.

BACKGROUND ART

Heretofore, a white-light emitting device constructed by combining agallium nitride (GaN) base light-emitting diode (LED) as semiconductorlight emitting device and phosphors as wavelength converting materialhas been noted as light source for a display or lighting system, makinguse of its advantages of low power consumption and long operating life.

As a typical light emitting device, a white LED comprising In-added GaNbase blue LED and Ce-activated yttrium aluminium garnet base yellowphosphor can be cited particularly. However, there are problems, asalready pointed, that the light amount in the range of red (600 nm orlonger) and blue green (480 nm to 510 nm) is small, and that the generalcolor rendering index Ra of the light from the light emitting device islow. Therefore, an improvement has been demanded.

To solve this problem, in Patent Document 1, it is disclosed that whiteLED emitting white composite light can be obtained by exciting, usingblue LED, phosphor consisting of a red phosphor such as(Ca_(1-a-b)Sr_(a)Eu_(b))S:Eu²⁺, which is used for increasing red lightcomponent in addition to the light component of a green phosphor such as(Y_(1-a-b)Gd_(a)Ce_(b))₃ (AL_(1-c)Ga_(c))₅O₁₂. It shows, in addition, amethod of obtaining white light by means of adjusting the weight ratioof the green phosphor to be 40% to 80% of a mixture of the green and thered phosphors. The red phosphor used here is a substance which can beexcited by the light emitted from a green phosphor. However, as theluminous efficiency of the red phosphor is lower than that of the greenphosphor, when the mixture having the above-mentioned combination andweight ratio of these phosphors is used, the weight ratio of the redphosphor is necessary to be 20% to 60%, which is relatively much. Thisthen leads to a problem of reduction in luminous flux emitted from thewhite LED because green light emitted from the green phosphor isabsorbed by the large amount of the red phosphor, having low luminousefficiency. Moreover, as the red phosphor used is lowmoisture-resistant, sulfide base red phosphor, there are problems ofease of deterioration, and of high production cost due to the difficultyin synthesis. This leads to the problems of the white LED, which isobtained by using the red phosphor, of low durability and highproduction cost. Furthermore, as the color of the emitted light from thegreen phosphor used is a little too yellowish, there is another problemthat the shortage of blue-green range of emitted light causes inferiorcolor rendering. In non-Patent Document 1, white LED, using SrGa₂S₄:Eu²⁺as green phosphor and ZnCdS:Ag,Cl as red phosphor, is disclosed. Thereare problems, with this LED too, of insufficient luminous flux,insufficient color rendering and vulnerability of the sulfide todeterioration when the white LED is used.

-   [Patent Document 1] Japanese Patent Laid-Open Publication (Kokai)    No. 2003-243715-   [Non-Patent Document 1] J. Electrochem. Soc. Vol. 150 (2003)pp.    H57-H60

DISCLOSURE OF THE INVENTION Problem to Be Solved by the Invention

The present invention has been made in view of the prior arts asmentioned above, aiming at creating a light emitting device which isexcellent in both luminance and color rendering. Therefore, the objectof the present invention is to provide a light emitting device which isexcellent in both luminance and color rendering, and a lighting systemand display using the same.

Means for Solving the Problem

The inventors of the present invention made an intensive investigationto solve the above problems, and found that, a light emitting devicehaving excellent luminance and excellent color rendering can be obtainedby using a mixture consisting of two or more kinds of phosphors whoseluminous efficiency are 35% or higher when excited by the visible lightfrom a semiconductor light emitting device at room temperature, whereinsaid mixture contains a first phosphor and a second phosphor that isdifferent from said first phosphor and capable of absorbing emittedlight from said first phosphor, and said first phosphor is contained 85weight % or more of said mixture of phosphors, which leads to creationof the present invention.

Advantageous Effect of the Invention

The present invention makes possible the creation of a light emittingdevice which is excellent in both luminance and color rendering. The useof the light emitting device of the present invention makes possible thecreation of a lighting system and a display which are excellent in bothemission efficiency and color rendering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating one example of alight emitting device of the present invention comprising phosphors aswavelength converting material and a semiconductor light emittingdevice.

FIG. 2 is a schematic cross-sectional view illustrating one example of asurface-emitting lighting system incorporated with the light emittingdevice shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be explained in detail referring toexamples. It should be borne in mind that the present invention is notlimited to the below-described examples and can be modified any wayinsofar as it does not depart from the scope of the present invention.

A light emitting device of the present invention is equipped withphosphors as wavelength converting material; and a semiconductor lightemitting device that emits visible light, wherein said phosphors are oneor more kinds of phosphors selected from a group consisting of oxides,oxynitrides and nitrides, and are a mixture consisting of two or morekinds of phosphors whose luminous efficiency are 35% or higher whenexcited by the visible light from said semiconductor light emittingdevice at room temperature; and said mixture contains a first phosphor,and a second phosphor that is different from said first phosphor andcapable of absorbing emitted light from said first phosphor, and saidfirst phosphor is contained 85 weight % or more of said mixture ofphosphors.

Material for the phosphors used in the present invention, is one or morekinds of phosphors selected from a group consisting of oxides,oxynitrides and nitrides. By using these materials, the light emittingdevice will not be vulnerable to deterioration when it is used, and willshow high emission efficiency when the temperature of the phosphorbecomes high under high-load light irradiation from, for example, apower LED. This leads to the preferable result of less vulnerability todeterioration and higher luminance. It is particularly preferable to useone or more kinds of phosphors selected from a group consisting ofinorganic oxides, inorganic oxynitrides and inorganic nitrides, as thelight emitting device is then quite hardly vulnerable to deteriorationwhen it is used.

In addition, phosphors used in the present invention are a mixtureconsisting of two or more kinds of phosphors whose luminous efficiencyare 35% or higher when excited by the visible light from saidsemiconductor light emitting device at room temperature. When a phosphorwith luminous efficiency of below 35% is used, even if the efficiency ofthe semiconductor light emitting device which excites the phosphors, ishigh, the emission efficiency of the entire light emitting device,obtained by combining these, will be low, which is not a preferableconsequence. The mixture of phosphors contains a first phosphor and asecond phosphor that is different from said first phosphor and capableof absorbing emitted light from said first phosphor. In particular, theluminous efficiency of the first phosphor is preferably 40% or higher,more preferably 45% or higher, and especially preferably 50% or higher,as the light emitted from the first phosphor is used for excitation ofthe second phosphor. The higher the luminous efficiency of the firstphosphor is, the better. Similarly, the higher the luminous efficiencyof the second phosphor is, the better. Actually, it is preferably 40% orhigher, more preferably 45% or higher, and especially preferably 50% orhigher.

In the following section, a method of calculating luminous efficiency,represented by the product of quantum absorption efficiency α_(q) andinternal quantum efficiency η_(i) will be described. First, the phosphorsample to be measured, in a state of powder or the like, is stuffed upin a cell with its surface smoothed enough to keep measurement accuracyto be high, and then it is set on a spectrophotometer having anintegrating sphere or the like. As the spectrophotometer, can be citedfor example MCPD2000 made by OTSUKA ELECTRONICS CO., LTD. The reason forthe use of an integrating sphere or the like is to count all the photonsboth reflected from the sample and released from the sample byphotoluminescence without fail, in other words, to prevent all thephotons from going outside of measurement system without being counted.A light source for exciting the phosphor is attached on thespectrophotometer. This light source, for example an Xe lamp, isadjusted using a filter or the like so that the emission peak wavelengthis 400 nm. By irradiating the sample to be measured with this light fromthe light source adjusted to have wavelength peak of 400 nm, theemission spectrum is measured. In the measured spectrum, there isactually also overlapped contribution of photons reflected from thesample, as well as photons produced from the sample by photoluminescenceinduced by the light from the excitation light source (hereinafter,called simply “excitation light”). Absorption efficiency α_(q) takes thevalue of N_(abs), the number of photons included in the excitation lightabsorbed in the sample, divided by N, the number of all the photons ofthe excitation light. First, as for the latter value N, which shows thenumber of all the photons of the excitation light, is calculated asfollows. The reflection spectrum I_(ref)(λ) is measured about a materialwhich is a measuring object installed on the spectrophotometer, and hasreflectance R of approx. 100% against the excitation light, for exampleSpectralon, a reflection plate made by Labsphere (having 98% ofreflectance against excitation light of 400 nm wavelength). The valuecalculated from the reflection spectrum I_(ref)(λ) following to (formula1 below is proportional to N.

[mathematical formula 1]

1/R∫λ·I_(ref)(λ)dλ  (formula 1)

In this formula, the integration may be performed at only such intervalsthat I_(ref)(λ) take substantially significant values. Then, as for theformer value N_(abs), is proportional to the amount calculated by(formula 2).

[mathematical formula 2]

1/R∫λ·I_(ref)(λ)dλ−∫λ·I(λ)dλ  (formula 2)

Here, the function I(λ) is a reflection spectrum in case the targetsample, whose α_(q) is intended to be decided, is set. The integrationintervals of (formula 2) are the same as those selected in (formula 1).By restricting the integration intervals as above, the second term of(formula 2) corresponds to the number of photons produced by thereflection of excitation light from the target sample, or in otherwords, corresponds to the number of all photons produced from the targetsample except for the number of photons produced by photoluminescenceinduced by the excitation light. As the actual measurement value of thespectrum is generally obtained as digital data which are divided by acertain finite band width which is related to λ, the integrations of(formula 1) and (formula 2) are calculated as finite sum, based on theband width. Consequently, α_(q) is calculated as the value of N_(abs)/N,which is equal to (formula 2)/(formula 1).

Next, a method of calculating internal quantum efficiency ƒ_(i) isexplained. The letter η_(i) takes the value of N_(PL), the number ofphotons produced by photoluminescence, divided by N_(abs), the number ofphotons absorbed in the sample. Here, N_(PL) is proportional to theamount calculated by (formula 3).

[mathematical formula 3]

∫λ·I(λ)dλ  (formula 3)

At this point, the integral interval is restricted to the wavelengthregion of photons that are produced from the sample by photoluminescenceso as to eliminate the contribution of photons, which are reflected fromthe sample, from the function I(λ). More concretely, the lower intervallimit of the (formula 3) integration takes the value of upper intervallimit of (formula 1) integration, and the upper interval limit takes thevalue of the range which is preferable to include the spectra originatedfrom photoluminescence. Consequently, η_(i) can be decided as (formula3)/(formula 2). Incidentally, the way to perform integration fromspectra of digital data is the same as in the case where α_(q) iscalculated.

Then, the luminous efficiency, defined in the present invention, isdecided as product of quantum absorption efficiency α_(q) and internalquantum efficiency η_(i), calculated in the way shown above.

It is preferable that the absorption efficiency of the second phosphor,at the wavelengths of light emitted from the semiconductor lightemitting device, is larger than the absorption efficiency of the secondphosphor, at the emission peak wavelength of the first phosphor. In suchinstance, the probability that the second phosphor emits light byexcitation of the light which is emitted from the semiconductor lightemitting device and absorbed in the second phosphor is higher than theprobability that the second phosphor emits light by excitation of thelight which is emitted from the first phosphor and absorbed in thesecond phosphor. This preferably leads to the result of obtaining alight emitting device having higher emission efficiency.

The light emitting device of the present invention, as mentionedpreviously, contains a mixture consisting of two or more kinds ofphosphor whose luminous efficiency is 35% of higher. The mixturecontains a first phosphor and a second phosphor that is different fromsaid first phosphor and capable of absorbing emitted light from saidfirst phosphor. At this point, the device contains said first phosphorwith 85 weight % or more, relative to said mixture of phosphor, or moreconcretely, relative to the sum of the first and second phosphors. Whenthe weight % of the first phosphor is below 85%, it is likely to createwhite LED having substantially much red in its color, instead ofobtaining white LED having high luminance and preferable white color. Toobtain more preferable white color, though it also depends on thebalance of the luminous efficiency between the first and secondphosphors or the absorption efficiency of the light emitted from thefirst phosphor into the second phosphor, the device preferably containsthe first phosphor with 89 weight % or more. Furthermore, to obtain moresolid white color, the weight % of the first phosphor is preferably 92weight % or more.

To obtain a light emitting device having high luminance and high colorrendering due to rich content of green and red light component, suchphosphors are usually selected that the emission peak wavelength L1 ofthe first phosphor is in the range of 490 nm≦L1≦550 nm and that theemission peak wavelength L2 of the second phosphor is in the range of600 nm≦L2≦700 nm. Further, it is preferable that the phosphors areselected so that the emission peak wavelength L1 of the first phosphoris in the range of 490 nm≦L1≦550 nm and that the emission peakwavelength L2 of the second phosphor is in the range of 600 nm≦L2≦700nm.

With the combination of phosphors selected as above, in case thesephosphors are excited by the light from a semiconductor light emittingdevice with peak wavelength of 380 nm to 480 nm in the visible lightrange, a light emitting device having almost all colors of emissionspectra and therefore high color rendering can be obtained. It showsespecially high luminance and color rendering, when the peak wavelengthof the semiconductor light emitting device is in the blue light range,which is from 420 nm to 480 nm. And it shows the highest luminance andcolor rendering, when the peak wavelength of the semiconductor lightemitting device is in the pure blue light range, which is from 435 nm to465 nm.

The general color rendering index Ra of the light from the lightemitting device of the present invention, releasing white light havinghigh luminance and high color rendering, is preferably 80 or larger, andparticularly preferably 85 or larger. With the most ideal combination ofsemiconductor light emitting device and phosphors, Ra is so high as tobe 88 or larger. Incidentally, the maximum value of Ra is 100.

When the emission peak wavelength L1 of the first phosphor is below orover the aforesaid range of 490 nm≦L1≦550 nm, or when the emission peakwavelength L2 of the second phosphor is below or over the aforesaidrange of 600 nm≦L2≦700 nm, the light emitting device obtained will be ofinferior luminance and color rendering, which is not preferable. For thesame reason, it is more preferable that the emission peak wavelength L1of the first phosphor is in the range of 500 nm≦L1≦540 nm, and that theemission peak wavelength L2 of the second phosphor is in the range of610 nm≦L≦670 nm. Moreover, it is particularly preferable that theemission peak wavelength L1 of the first phosphor is in the range of 510nm≦L1≦540 nm, and that the emission peak wavelength L2 of the secondphosphor is in the range of 620 nm≦L2≦660 nm, because the colorreproduction range thereof will be large when used for display, as wellas having intensified green or red light, high luminance and high colorrendering.

In the following, examples of first phosphor and second phosphor usedfor the light emitting device of the present invention, will bedescribed. It should be noted that phosphors are not limited to thoseexemplified in the following section.

As a first phosphor, can be cited the phosphor which contains at leastCe as luminescent center ion in the host crystal described by thegeneral formula (1) or (2) below. It is particularly preferable that thefirst phosphor contains at least one of the phosphors, from thestandpoint of obtaining a light emitting device having high luminanceand color rendering, because of its less vulnerability to deteriorationat the point of use and less variation of luminance according to thetemperature variation of the light emitting device when used.

M¹ _(a)M² _(b)M³ _(c)O_(d)  (1)

M⁴ _(e)M⁵ _(f)O_(g)  (2)

In the following, the general formula (1) above will be described.

M¹, M², and M³ represent at least one bivalent metal element, at leastone trivalent metal element and at least one tetravalent metal element,respectively, and a, b, c and d indicate values in the range shownbelow.

2.7≦a≦3.3

1.8≦b≦2.2

2.7≦c≦3.3

11.0≦d≦13.0

In the formula (1) above, M¹ is at least one bivalent metal element. Inview of light luminous efficiency or the like, it is preferably at leastone of elements selected from the group consisting of Mg, Ca, Zn, Sr, Cdand Ba, more preferably at least one element selected from Mg, Ca andZn. Ca is particularly preferable. In this instance, Ca can be usedeither singly or in combination with Mg. In principle, M¹ should consistof the elements referred to above as preferable. However, it can containother bivalent metal elements, as far as performance is not impaired.

In the formula (1) above, M² is at least one trivalent metal element. Inview of the same aspects as above, it is preferably at least one ofelements selected from the group consisting of Al, Sc, Ga, Y, In, La, Gdand Lu, more preferably at least one element selected from Al, Sc, Y andLu. Sc is particularly preferable. In this instance, Sc can be usedeither singly or in combination with Y or Lu. In principle, M² shouldconsist of the elements referred to above as preferable. However, it cancontain other trivalent metal elements, as far as performance is notimpaired.

In the formula (1) above, M³ is at least one tetravalent metal element.In view of the same aspects as above, it is preferable that M³ containsSi as a minimum requirement. The content of Si in the tetravalent metalelement shown as M³ is usually 50 mole % or more, preferably 70 mole %or more, more preferably 80 mole % or more, far more preferably 90 mole% or more. Apart from Si, tetravalent metal element M³ is preferably atleast one of element selected from the group consisting of Ti, Ge, Zr,Sn and Hf, more preferably at least one element selected from the groupconsisting of Ti, Zr, Sn and Hf. Of these, Sn is particularly preferred.The particularly preferred M³ element is Si. In principle, M³ shouldconsist of the elements referred to above as preferable. However, it cancontain other tetravalent metal elements, as far as performance is notimpaired.

In the present invention, performance is deemed not impaired if thecontent of other elements, relative to above-mentioned M¹, M² and M³, is10 mole % or lower, preferably 5 mole % or lower, more preferably 1 mole% or lower.

In the formula (1) above, a, b, c and d fall within the following range:2.7≦a≦3.3, 1.8≦b≦2.2, 2.7≦c≦3.3 and 11.0≦d≦13.0, respectively. a, b, cand d of the present phosphor may deviate within the above range, insuch case that the element constituting the luminescent center ionoccupies the position of the crystal lattice of one of the metal ions ofM¹, M², and M³, or that it is located in the interstitial gap of thecrystal lattice. However the crystal structure of the present phosphorsis one of the garnet crystal structures. This is usually a body centeredcubic lattice crystal structure where a, b, c and d represent the valueof 3,2,3 and 12, respectively.

The luminescent center ion, contained in the host material of thecrystal structure, is required to contain at least Ce. For the fineadjustment of its luminescence property, it may contain, as coactivatoragent, at least one type of divalent to tetravalent element selectedfrom the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Pr, Nd, Sm, Eu, Tb,Dy, Ho, Er, Tm and Yb. Particularly, it may contain at least one type ofdivalent to tetravalent element selected from the group consisting ofMn, Fe, Co, Ni, Cu, Sm, Eu, Tb, Dy and Yb. Divalent Mn, divalent ortrivalent Eu, or trivalent Tb may be preferably added. In casecoactivator agent is contained, the amount of the coactivator agent isusually 0.01 mol to 20 mol relative to 1 mol of Ce.

In case the concentration of Ce, which functions as activator agent, istoo low, there is too little activator agent, which emits light, andthis may result in the lowering of emission intensity. On the otherhand, if the concentration is too high, the extent of concentrationquenching may be heightened, resulting in the lowering of emissionintensity. In terms of emission intensity, the concentration of Ce is,in molar ratio, preferably in the range of 0.0001 to 0.3, relative to 1mol of M¹. It is more preferably in the range of 0.001 to 0.1, and farmore preferably in the range of 0.005 to 0.05.

In the following, the general formula (2) below will be described.

M⁴ _(e)M⁵ _(f)O_(g)  (2)

M⁴ and M⁵ in the Formula (2) represent at least one bivalent metalelement and at least one trivalent metal element, respectively, and e, fand g indicate values in the range shown below, respectively.

0.9≦e≦1.1

1.8≦f≦2.2

3.6≦g≦4.4

In the formula (2) above, M⁴ is at least one bivalent metal element. Inview of light luminous efficiency or the like, it is preferably at leastone type of element selected from the group consisting of Mg, Ca, Zn,Sr, Cd and Ba, more preferably one element selected from Mg, Ca and Zn.Ca is particularly preferable. In this instance, Ca can be used eithersingly or in combination with Mg. In principle, M⁴ should consist of theelements referred to above as preferable. However, it can contain otherbivalent metal elements, as far as performance is not impaired.

In the formula (2) above, M⁵ is a trivalent metal element. In view ofthe same aspects as above, it is preferably at least one of elementsselected from the group consisting of Al, Sc, Ga, Y, In, La, Gd and Lu,more preferably at least one element selected from Al, Sc, Y and Lu. Scis particularly preferable. In this instance, Sc can be used eithersingly or in combination with Y or Lu. In principle, M⁵ should consistof the elements referred to above as preferable. However, it can containother trivalent metal elements, as far as performance is not impaired.

In the present invention, performance is deemed not impaired if thecontent of other elements, relative to above-mentioned M⁴ and M⁵, is 10mole % or lower, preferably 5 mole % or lower, more preferably 1 mole %or lower.

It is preferable that the element ratio is in the range described below,from the standpoint of luminescence property. Especially with respect tothe concentration of Ce, which functions as activator agent, in case itis too low, there is too little activator agent, which emits light, andthis may result in the lowering of emission intensity. On the otherhand, if the concentration is too high, the extent of concentrationquenching may be heightened, resulting in the lowering of emissionintensity. In terms of emission intensity, the concentration of Ce is,in molar ratio, preferably in the range of 0.0001 to 0.3, relative to 1mol of M⁴. It is more preferably in the range of 0.001 to 0.1, and farmore preferably in the range of 0.005 to 0.05.

The luminescent center ion, contained in the host material of thecrystal structure, is required to contain at least Ce. For the fineadjustment of its luminescence property, it may contain, as coactivatoragent, at least one type of divalent to tetravalent element selectedfrom the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Pr, Nd, Sm, Eu, Tb,Dy, Ho, Er, Tm and Yb. Particularly, it may contain at least one type ofdivalent to tetravalent element selected from the group consisting ofMn, Fe, Co, Ni, Cu, Sm, Eu, Tb, Dy and Yb. Divalent Mn, divalent ortrivalent Eu, or trivalent Tb may be preferably added. In casecoactivator agent is contained, the amount of the coactivator agent isusually 0.01 mol to 20 mol relative to 1 mol of Ce.

In the following section, second phosphor will be described. Thoughthere is no special limitation on second phosphor, as far as it canabsorb the light emitted from the above-mentioned first phosphor, it isparticularly preferable for obtaining light emitting device having highluminance and color rendering that it contains at least a compositionincluding M element, A element, D element, E element and X element (Mrepresents one or more than one elements selected from the groupconsisting of Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb. Itcontains Eu as a minimum requirement. A represents one or more than oneelements selected from the group consisting of bivalent metal elementsother than M element. D represents one or more than one elementsselected from the group consisting of tetravalent metal elements. Erepresents one or more than one elements selected from trivalent metalelements. X represents one or more than one elements selected from thegroup consisting of O, N, and F), because of its less vulnerability todeterioration at the point of use and less variation of luminanceaccording to the temperature variation of the light emitting device whenused.

At this point, M represents one or more than one elements selected fromthe group consisting of Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm andYb. It contains Eu as a minimum requirement. More preferably, Mrepresents one or more than one elements selected from the groupconsisting of Mn, Ce, Sm, Eu, Tb, Dy, Er, and Yb. Eu is the particularlypreferable element.

A represents one or more than one elements selected from the groupconsisting of bivalent metal elements other than M. Preferably, itrepresents one or more than one elements selected from the groupconsisting of Mg, Ca, Sr and Ba. Particularly preferable is Ca.

D represents one or more than one elements selected from the groupconsisting of tetravalent metal elements. Preferably, it represents oneor more than one elements selected from the group consisting of Si, Ge,Sn, Ti, Zr and Hf. Si is the particularly preferable element.

E represents one or more than one elements selected from the groupconsisting of trivalent metal elements. Preferably, it represents one ormore than one elements selected from the group consisting of B, Al, Ga,In, Sc, Y, La, Gd and Lu. Al is the particularly preferable element.

X represents one or more than one elements selected from the groupconsisting of O, N, and F. Particularly preferred is N or a combinationof N and O.

A concrete composition of the above-described composition can berepresented by general formula (3) below, for example.

M_(a)A_(b)D_(c)E_(d)X_(e)  (3)

In the formula (3) above, a, b, c, d and e indicate a value satisfyingall the conditions shown below.

0.00001≦a≦0.1  (i)

a+b=1  (ii)

0.5≦c≦4  (iii)

0.5≦d≦8  (iv)

0.8×(⅔+ 4/3×c+d)≦e  (v)

e≦1.2×(⅔+ 4/3×c+d)  (vi)

Here, index a represents the addition amount of M element which playsthe role of luminescent center. It is preferable to set index a, theratio of the number of M atom to the number of (M+A) element in thephosphor (a=(number of M atom/(number of M atom+number of A atom)), at0.00001 or larger, and 0.1 or smaller. If the value of a is smaller than0.00001, the number of M element constituting the luminescent center issmall and this results in the lowering of emission luminance. If thevalue of a is larger than 0.1, concentration quenching due tointeraction among M ions themselves may occur, resulting in the loweringof luminance.

In case the M element is Eu, it is preferable that the value a is set inthe range of 0.002 or larger, and 0.03 or smaller, as it can have highemission luminance then.

The value c represents the content of D element such as Si, and falls inthe range of 0.5≦c≦4. Preferably, the value of c is in the range0.5≦c≦1.8, more preferably c is 1. Emission luminance decreases in casethe value of c is smaller than 0.5, and in case it is larger than 4. Inthe range 0.5≦c≦1.8, emission luminance is high. It is particularly highin case the value of c is 1.

The value d represents the content of E element such as Al and falls inthe range of 0.5≦c≦18. Preferably, the value of d is in the range0.5≦d≦1.8, more preferably d is 1. Emission luminance decreases in casethe value of d is smaller than 0.5, and in case it is larger than 8. Inthe range 0.5≦d≦1.8, emission luminance is high. It is particularly highin case the value of d is 1.

The value e represents the content of X element such as N. The value ofe is larger than or equal to 0.8×{(⅔)+( 4/3)×c+d}, and smaller than orequal to 1.2×{(⅔)+( 4/3)×c+d}. It is more preferable that e takes thevalue of 3. In case the value deviates outside the above range, emissionluminance decreases.

Of the compositions mentioned above, the preferable composition ensuringhigh emission luminance is such that it contains Eu as M element, Ca asA element, Si as D element, Al as E element, and N as X element as aminimum requirement. It is particularly preferable that it is aninorganic compound in which M element is Eu, A element is Ca, D elementis Si, E element is Al, and X element is N or a mixture of N and O.

In case X element is N or a mixture of N and O, too large value of(number of moles of O)/(number of moles of N+number of moles O) lowersemission intensity. In terms of emission intensity, the value of (numberof moles of O)/(number of moles of N+number of moles O) is preferably0.5 or smaller, and more preferably 0.3 or smaller. It is far morepreferably 0.1 or smaller, as the red phosphor can emit light withemission peak wavelength of 640 nm to 660 nm, which means excellentcolor purity. From another viewpoint, it is also preferable to set thevalue of (number of moles of O)/(number of moles of N+number of moles O)to be 0.1 to 0.3, as the emission peak wavelength can be adjusted in therange of 600 nm to 640 nm, which is close to the wavelength region withhigh visual sensitivity by human, and therefore, a light emitting devicehaving high luminance can be obtained.

With respect to the mixture of phosphors, on which is irradiated withthe light from semiconductor light emitting device, it is preferable toadjust the mixing ratio between first phosphor and second phosphorincrementally or continuously, depending on the distance from thesemiconductor light emitting device, in such a manner that the longerthe distance is, the higher the mixing ratio of the first phosphor is.

In other words, when the first and second phosphors are mixed anddisposed at the portion where the light from the semiconductor lightemitting device is directly irradiated, it is preferable that the firstphosphor is contained in the mixture of phosphors with relatively lowmixing ratio where it is close to the semiconductor light emittingdevice and the light from the semiconductor light emitting device isdirectly irradiated, and that the first phosphor is contained in themixture of phosphor with relatively high mixing ratio where it is farfrom the semiconductor light emitting device and the light from it isnot directly irradiated. With this gradation of the mixing ratio ofphosphors, a part of the light from the semiconductor light-emitting canbe absorbed first into the portion with large amount of second phosphorcontained and excite the second phosphor intensively, and then the restpart of the light, which is not absorbed in the second phosphor, canexcite the phosphor mixture with more mixing ratio of first phosphor.This leads to high luminous efficiency of the entire phosphor.

To obtain a light emitting device that can release white light with highluminance and color rendering, it is preferable that the device containsa mixture of phosphors wherein the color coordinate of the emitted lightfrom said mixture of phosphors, when it is irradiated by the emittedlight from said semiconductor light emitting device, is in the rangesurrounded by the quadrangle with apexes having CIE color coordinates(0.450, 0.350), (0.550, 0.450), (0.400, 0.600), and (0.300, 0.500). If amixture of phosphors whose color coordinate is out of this range is usedand it is incorporated with a semiconductor light emitting deviceemitting blue light, it is difficult to obtain white light. And toobtain a light emitting device emitting more solid white light, thecolor coordinate of the emitted light from said mixture of phosphors ismore preferably in the range surrounded by the quadrangle with apexes(0.500, 0.400), (0.550, 0.450), (0.400, 0.600), and (0.320, 0.520), andit is far more preferably in the range surrounded by the quadrangle withapexes (0.480, 0.420), (0.520, 0.480), (0.410, 0.590), and (0.340,0.520).

Moreover, in the present light emitting device, in order to obtain whiterange of light having high luminance, high chromaticity and high colorrendering, it is preferable to adjust the emission wavelength of thesemiconductor light emitting device, mixing ratio of the phosphors andcoating amount of the phosphor onto semiconductor light emitting devicein such a manner that the color coordinate of a composite lightconsisting of emitted light from said semiconductor light emittingdevice and emitted light from said mixture of phosphors, which isexcited by the emitted light from said semiconductor light emittingdevice, is in the range surrounded by the quadrangle with apexes havingCIE color coordinates (0.275, 0.175), (0.450, 0.400), (0.350, 0.450),and (0.175, 0.250). For similar reason, it is more preferable to adjustthe emission wavelength of the semiconductor light emitting device,mixing ratio of the phosphors and coating amount of the phosphors insuch a manner that the color coordinate of the composite light of thelight emitting device is in the range surrounded by the quadrangle withapexes having color coordinates (0.278, 0.210), (0.410, 0.385), (0.353,0.420) and (0.215, 0.265), far more preferably, it is in the rangesurrounded by the quadrangle with apexes having color coordinates(0.280, 0.250), (0.370, 0.370), (0.355, 0.390) and (0.255, 0.275), andmost preferably, it is in the range surrounded by the quadrangle withapexes having color coordinates (0.295, 0.275), (0.340, 0.330), (0.330,0.340) and (0.285, 0.295).

The light emitting device of the present invention contains at least twokinds of phosphors as wavelength converting material and at least onesemiconductor light emitting device that emits visible light, forexample LED, LD or the like, and it contains phosphors that absorbvisible light emitted from the semiconductor light emitting device andemits visible light with longer wavelengths, which leads to highluminance and color rendering. Consequently, it can be preferably usedas light source of display such as color liquid crystal display,lighting systems such as surface emitting type, or the like.

In the following, light emitting device of the present invention isexplained referring to the drawings. FIG. 1 is a schematiccross-sectional view illustrating one example of a light emitting devicecomprising phosphors as wavelength converting material and asemiconductor light emitting device. FIG. 2 is a schematiccross-sectional view illustrating one example of a surface-emittinglighting system incorporated with the light emitting device shown inFIG. 1. In FIG. 1 and FIG. 2, the letter 1 shows light emitting device,2 shows mount lead, 3 shows inner lead, shows semiconductor lightemitting device, 5 shows phosphor-containing resin portion, 6 showsconductive wire, 7 shows molded member, 8 shows surface-emittinglighting system, 9 shows diffusion plate and 10 shows support case.

The light emitting device 1 of the present invention is in a common,shell type, as illustrated in FIG. 1. In a cup-like portion at the topof mount lead 2, semiconductor light emitting device 4 consisting of aGaN base blue light-emitting diode is fixed by being covered withphosphor-containing resin portion 5. The phosphor-containing resinportion 5 is formed by mixing and dispersing phosphor of the presentinvention into a binder, such as epoxy resin, acrylic resin or the like,and poring it into the cup-like portion. On the other hand,semiconductor light emitting device 4 and mount lead 2 are electricallyconnected through mounting member such as silver paste, andsemiconductor light emitting device 4 and inner lead are electricallyconnected through conductive wire 6. All of these members are coveredand protected with molded member 7, formed from epoxy resin or the like.

Surface-emitting lighting system 8 incorporated with this light emittingdevice 1 is constructed, as shown in FIG. 2, so that a lot of lightemitting devices 1 are disposed on the bottom of rectangular supportcase 10, whose inner surfaces are light-impermeable ones such as whiteand flat surfaces, provided with power sources, circuits or the like(not shown in the drawings) for driving light emitting device 1 on outerends of the devices 1, and that diffusion plate 9, such as an opalescentacrylic plate, is fixed at the portion which corresponds to the lid ofsupport case 10 so as to emit light uniformly.

When this surface-emitting lighting system 8 is driven, the voltageapplied to semiconductor light emitting device 4 of light emittingdevice 1 makes light such as blue light be emitted, and then a part ofthe emitted light is absorbed in the mixture of phosphors whichfunctions as wavelength converting material in phosphor-containing resinportion 5, leading to emission of light with longer wavelengths. On theother hand, this light emitted, mixed with light, such as the bluelight, which is not absorbed in the phosphors, produce light with highcolor rendering. This light is released through diffusion plate 9 in theupward direction in the drawing, leading to realizing illuminating lightwhich has uniform luminance over the plane of diffusion plate 9 ofsupport case 10.

The present invention will be explained in further detail belowreferring to examples. It is to be understood that the present inventionis not limited to specific examples explained below as far as it is notdeparted from the scope thereof.

Example 1

A first phosphor and a second phosphor were mixed, with weight % of 94and 6 respectively, to create a mixture of phosphors. The first phosphorused, represented by the chemical composition of Ca₃Sc₂Si₃O₁₂, was aphosphor of an oxide, which has luminous efficiency of 46% and emissionpeak wavelength of 505 nm, and includes 0.03 mol (0.01 mol relative to 1mol of Ca in the chemical composition) of Ce as activator agent. Thesecond phosphor used, represented by the chemical composition ofCaAlSiN₃, was a phosphor of a nitride, which has luminous efficiency of54% and emission peak wavelength of 650 nm, and includes 0.01 mol of Euas activator agent.

This intensive emission peak observed in the emission spectrum of thefirst phosphor is sufficiently overlapped with wavelength of theexcitation band observed in the excitation spectrum of the secondphosphor. Therefore, it was confirmed that the light emitted from thefirst phosphor was absorbed in the second phosphor and excited thesecond phosphor.

Then, this mixture of phosphor was irradiated with blue light emittedfrom In added GaN base semiconductor light emitting device with emissionpeak wavelength of 460 nm, this resulted in that the phosphor emittedlight with CIE color coordinate (x, y) of (0.420, 0.500).

Furthermore, shell type white LED was produced by the followingprocedure. First, an LED (C460XT, made by Cree), which emits light at460 nm of wavelength, was mounted on a cup-like portion of a frame ofthe shell type LED using a conductive silver paste as mounting member.Then, the electrode of the LED and inner lead were connected by bondingusing Au wire. Then, a mixture of phosphors and resin (hereinafter, itis called “phosphor paste”) was prepared by mixing well the abovedescribed phosphors and epoxy resin with the ratio of 1 gram to 10grams. The paste was poured in the cup-like portion of the frame wherethe LED was mounted. By maintaining this at 120° C. for an hour, theepoxy resin was cured. Next, the frame with the LED and the phosphorsinstalled as mentioned above was inserted in a shell type mold havingepoxy resin poured therein, and then it was maintained at 120° C. for anhour. After the resin was cured, it was demolded, and consequently,shell type white LED was obtained.

Electric current of 20 mA was supplied to the white LED obtained asabove at room temperature (approx. 24° C.). All the light emitted fromthe white LED was measured for the emission spectrum, by receiving allof it with an integrating sphere and guiding it to a spectroscopethrough an optical fiber. The data of the emission spectrum wererecorded as numerical values of emission intensity at 5 nm intervals inthe range from 380 nm to 780 nm. In the result, the white LED showedhigh luminescence property including color temperature of 6800K, CIEcolor coordinates (x, y) of (0.309, 0.318), general color renderingindex Ra of 90 and whole luminous flux of 2.5 lm.

This white LED showed excellent light emission with outstandingly highgeneral color rendering index and whole luminous flux compared to theconventional product of pseudo-white LED, which is constructed bycombining blue LED and yttrium aluminium garnet base phosphor and hasgeneral color rendering index of 79 and whole luminous flux of 1.9 lm.

Therefore, it was found that the use of light emitting device of thepresent invention makes it possible to create a display having highluminance and wide color reproduction range relative to the conventionalproducts, and a lighting system having high luminance and colorrendering.

Example 2

A first phosphor with weight % of 95 and a second phosphor were mixed,to create a mixture of phosphors. The first phosphor used, representedby the chemical composition of CaSc₂O₄, was a phosphor of an oxide,which has luminous efficiency of 43% and emission peak wavelength of 516nm, and includes 0.01 mol of Ce as activator agent. The second phosphorused, represented by the chemical composition of CaAlSiN₃, was aphosphor of a nitride, which has luminous efficiency of 54% and emissionpeak wavelength of 650 nm, and includes 0.01 mol of Eu as activatoragent.

This intensive emission peak observed in the emission spectrum of thefirst phosphor is sufficiently overlapped with wavelength of theexcitation band observed in the excitation spectrum of the secondphosphor. Therefore, it was confirmed that the light emitted from thefirst phosphor was absorbed in the second phosphor and excited thesecond phosphor.

Then, this mixture of phosphor was irradiated with blue light emittedfrom In added GaN base semiconductor light emitting device withwavelength of 460 nm, this resulted in that the phosphor emitted lightwith CIE color coordinate (x, y) of (0.420, 0.495).

Furthermore, shell type white LED was produced by the same procedure asshown in example 1, and the luminescence property was measured.

In the result, this white LED showed the luminescence property includingcolor temperature of 6400K, CIE color coordinates (x, y) of (0.320,0.320), general color rendering index Ra of 89 and whole luminous fluxof 2.3 lm. Of these, general color rendering index and whole luminousflux were outstandingly high compared to the conventional pseudo-whiteLED, this meant that excellent light emission could be obtained.

Therefore, it was found that the use of light emitting device of thepresent invention makes it possible to create a display having highluminance and wide color reproduction range relative to the conventionalproducts and a lighting system having high luminance and high colorrendering.

INDUSTRIAL APPLICABILITY

The present invention can be applied in any field where light isinvolved. It is preferably used, for example, as lighting system usedindoors as well as outdoors, and a display for various electronicappliances such as cellular phone, electric appliances for householduse, display to be installed outdoors.

Although the present invention was explained in detail referring tocertain embodiments, it is evident for those skilled in the art thatvarious changes or modifications can be made thereto without departingfrom the spirit and scope of the present invention.

The present invention is based on the specification of Japanese PatentApplication No. 2004-194509 filed on Jun. 30, 2004, and its entirety ishereby included by reference.

1. A light emitting device comprising: phosphors as wavelength converting material; and at least one semiconductor light emitting device that emits visible light, wherein said phosphors are independently selected from the group consisting of oxides, oxynitrides and nitrides, and are a mixture consisting of two or more phosphors, each of whose luminous efficiency is 35% or higher when excited by the visible light from said semiconductor light emitting device at room temperature; and said mixture contains a first phosphor, and a second phosphor that is different from said first phosphor and capable of absorbing emitted light from said first phosphor, and said first phosphor is contained 85 weight % or more of said mixture of phosphors, wherein the second phosphor has the following formula (3): M_(a)A_(b)D_(c)E_(d)X_(e)  (3) wherein M represents Eu alone or with one or more elements selected from the group consisting of Mn, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm and Yb; A represents one or more elements selected from the group consisting of bivalent metal elements other than M; D represents one or more elements selected from the group consisting of tetravalent metal elements; E represents one or more elements selected from the group consisting of trivalent metal elements; X represents one or more elements selected from the group consisting of O, N, and F, and the values of a, b, c and d are selected from values meeting the following conditions (i) through (iv): 0.00001≦a≦0.1  (i), a+b=1  (ii) c=d=1  (iii) e=3  (iv).
 2. A light emitting device as defined in claim 1, wherein the emission peak wavelength L1 of said first phosphor is in the range of 490 nm≦L1≦550 nm and the emission peak wavelength L2 of said second phosphor is in the range of 600 nm≦L2≦700 nm.
 3. A light emitting device as defined in claim 1, wherein said mixture of phosphors exhibits a color coordinate (x, y) of the emitted light from said mixture of phosphors, when it is irradiated by the emitted light from said semiconductor light emitting device, in the range surrounded by the quadrangle with apexes having CIE color coordinates (0.450, 0.350), (0.550, 0.450), (0.400, 0.600), and (0.300, 0.500).
 4. A light emitting device as defined in claim 1, wherein the color coordinate of a composite light consisting of emitted light from said semiconductor light emitting device and emitted light from said mixture of phosphors that is excited by the emitted light from said semiconductor light emitting device, is in the range surrounded by the quadrangle with apexes having CIE color coordinates (0.275, 0.175), (0.450, 0.400), (0.350, 0.450), and (0.175, 0.250).
 5. A lighting system, wherein said lighting system uses said light emitting device as defined in claim
 1. 6. A display, wherein said display uses said light emitting device as defined in claim
 1. 7. A light emitting device as defined in claim 1, wherein A represents one or more elements selected from the group consisting of Mg, Ca, Sr and Ba.
 8. A light emitting device as defined in claim 1, wherein A represents one or more elements selected from the group consisting of Mg, Ca, Sr and Ba; D represents one or more elements selected from the group consisting of Si, Ge, Sn, Ti, Zr and Hf; and E represents one or more elements selected from the group consisting of B, Al, Ga, In, Sc, Y, La, Gd and Lu.
 9. A light emitting device as defined in claim 1, wherein M includes at least Eu; A includes at least Ca; D includes at least Si; E includes at least Al; and X includes at least N.
 10. A light emitting device as defined in claim 1, wherein M represents Eu; A represents Ca; D represents Si; E represents Al; and X represents N or a mixture of N and O.
 11. A phosphor mixture, comprising: CaAlSiN₃:Eu; and Ca₃Sc₂Si₃O₁₂:Ce; wherein: the mixture comprises Ca₃Sc₂Si₃O₁₂:Ce in an amount of 94 wt % based on a total weight of the phosphor mixture; and the mixture comprises CaAlSiN₃:Eu in an amount of 6 wt % based on a total weight of the phosphor mixture.
 12. Lighting equipment, comprising: the phosphor mixture according to claim 11; and an LED.
 13. A phosphor mixture, comprising: a first phosphor having a composition given by the formula M_(a)A_(b)D_(c)E_(d)X_(e), wherein: M comprises at least one element selected from the group consisting of Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb; A comprises at least one element selected from the group consisting of divalent metal elements other than M; D comprises at least one element selected from the group consisting of tetravalent metal elements; E comprises at least one element selected from the group consisting of trivalent metal elements; and X comprises at least one of O and N; wherein: a+b=1; c=d=1; 0.8×(⅔+ 4/3×c+d)≦e; and e≦1.2×(⅔+ 4/3×c+d); and a second phosphor having an emission peak at a wavelength of from 490 nm to 550 nm when irradiated with excitation light having a wavelength of from 420 to 500 nm.
 14. The phosphor mixture according to claim 13, wherein: A comprises at least one member selected from the group consisting of Mg, Ca, Sr and Ba; E comprises at least one element selected from the group consisting of Al and Ga; and D comprises at least one element selected from the group consisting of Si and Ge.
 15. The phosphor mixture according to claim 13, wherein Z comprises Eu.
 16. The phosphor mixture according to claim 13, wherein the first phosphor comprises CaAlSiN₃:Eu.
 17. The phosphor mixture according to claim 13, wherein the second phosphor comprises Ca₃Sc₂Si₃O₁₂:Ce
 18. The phosphor mixture according to claim 13, wherein: the first phosphor comprises CaAlSiN₃:Eu; and the second phosphor comprises Ca₃Sc₂Si₃O₁₂:Ce.
 19. The phosphor mixture according to claim 13, wherein the second phosphor comprises a phosphor having a garnet crystal structure with Ce as an activator.
 20. The phosphor mixture according to claim 19, wherein the second phosphor comprises an Al garnet phosphor containing Y and/or Tb.
 21. The phosphor mixture according to claim 19, wherein the second phosphor comprises an Si garnet phosphor containing Sc.
 22. The phosphor mixture according to claim 19, wherein a chromaticity of an emission spectrum of the mixture satisfies x=0.420, and y=0.500, when excited with an excitation light having a wavelength of 460 nm.
 23. The phosphor mixture according to claim 19, wherein a chromaticity of an emission spectrum of the mixture satisfies x=0.420, and y=0.495, when excited with an excitation light having a wavelength of 460 nm.
 24. A light emitting device, comprising: the phosphor mixture according to claim 13; and a light emitting part emitting light that emits light in a wavelength range of from 380 nm to 480 nm.
 25. The light emitting device according to claim 24, wherein the light emitting part comprises an LED.
 26. The light emitting device according to claim 25, wherein the LED comprises Ga.
 27. The light emitting device according to claim 24, wherein a general color rendering index Ra of the light emitting device is at least
 80. 28. The light emitting device according to claim 24, wherein a correlated color temperature of the light emitting device is 6400K.
 29. The light emitting device according to claim 24, wherein a correlated color temperature of the light emitting device is 6800K. 